With respect to developing systems used for the image-forming apparatus of an electrophotographic system, a mono-component developing system using only toner as a main component of a developer and a two-component developing system using toner and carrier as main components of a developer have been known.
A developing device of the mono-component developing system is provided with a toner supporting member that supports toner thereon and transports the toner and a frictional charging member that is made in contact with the toner supporting face of the toner supporting member. Upon passing through the contact position with the frictional charging member, the toner, supported on the toner supporting member, is made friction-contact with the frictional charging member to be formed into a thin film, and also charged to a predetermined polarity. In this manner, since the mono-component developing device carries out a toner-charging process through the friction-contact with the frictional charging member, the resulting advantage is that the structure is simple, small and inexpensive. However, toner degradation tends to occur due to a high stress at the contact position with the frictional charging member, and the toner chargeability tends to be impaired in a comparatively early stage. Due to the contact pressure between the toner supporting member and the frictional charging member, the toner adheres to the two members to cause degradation in the capability of charging the toner, with the result that the service life of the developing device is shortened comparatively.
Since the developing device of the two-component developing system charges the toner and the carrier to predetermined polarities by making them in friction-contact with each other, the stress to be applied to the toner is smaller in comparison with that of the mono-component developing device. Since the surface area of the carrier is larger in comparison with that of the toner, the carrier is less vulnerable to contamination due to adhesion of the toner. However, since stains (spent), that is, adhesion of toner fine fragments to the surface of the carrier particle, tend to occur due to a long-term use, the toner-charging capability is consequently lowered to cause problems of fogging and toner scattering. An attempt may be made to increase the amount of the carrier to be housed in the developing device so as to prolong the service life of the two-component developing device; however, this causes a large size of the developing device.
In order to solve the above-mentioned problems relating to the two-component developing device, a developing system has been reported in which the carrier or the carrier and the toner are supplied on demand, while the developer whose charging capability has been lowered is collected (Japanese Patent-Application Laid-Open No. 59-100471). In accordance with this technique, the life of the developer can be prolonged without the necessity of a large-size developing device. However, another mechanism for collecting the discharged carrier is required. The consumption of the carrier increases, resulting in problems of costs and the environment. Furthermore, a predetermined amount of printing processes need to be carried out until the rate of undeteriorated carrier and deteriorated carrier has been stabilized.
Japanese Patent-Application Laid-Open No. 2003-287959 has proposed a hybrid developing system in which, from a developer containing a toner and a carrier, held on the outer circumferential face of a magnetic roller, only the toner is selectively supplied onto the outer circumferential face of a developing roller, and by using only the toner held on the outer circumferential face of this developing roller, an electrostatic latent image (electrostatic latent image portion) on a photosensitive member is developed. In such a developing system, reverse polarity particles that are charged to a polarity reversed to the charge polarity of the toner when made in friction-contact with the carrier are added to the developer so as to adhere to the carrier. Thus, the corresponding reverse polarity particles serve as charging sites for the carrier so that the toner charging capability of the carrier is ensured, and the carrier deterioration can be suppressed.
In the above-mentioned hybrid developing system, however, the toner holding time is comparatively long, with the result that, for example, when an image with a small image area ratio (white/black ratio), such as a character image, is continuously printed, the reverse polarity particles are progressively buried into the toner particle, failing to obtain a stable toner developing property for a long period. The resulting problem is that, upon executing endurance printing processes, the image density is lowered.
In an attempt to further externally add particles having the same polarity as that of the charge polarity of toner particles, since the corresponding same polarity particles are not allowed to effectively adhere to, and secured onto the toner particle, it is also not possible to obtain a stable toner developing property for a long period, and upon executing endurance printing processes, the image density is lowered.